This section introduces aspects that may facilitate better understanding of the present disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
In a typical cellular radio system, end-user radio terminals, also known as user terminals, mobile stations or user equipment, UE, are arranged to communicate via a radio access network (RAN) to one or more core networks. The RAN covers a geographical area which is divided into cells each being served by a base station, which may be referred to as NodeB, evolved NodeB (eNodeB), or eNB, depending on protocols and technologies.
One of the most basic requirements for any cellular radio system is the possibility for a UE to initiate a connection request, commonly referred to as random access. In 3GPP LTE (3rd Generation Partnership Project Long Term Evolution), as specified in 3GPP TS (Technical Specification) 36.211 v12.4.0, the random access procedure comes in two forms, allowing access to be either contention-based or contention-free. The contention-based procedure consists of four-steps, as specified in 3GPP TS 36.321 v12.4.0, including: Step 1, Preamble transmission; Step 2, Random access response; Step 3, Layer 2/Layer 3 message transmission; and Step 4, Contention resolution message. The contention-free random access procedure is typically used in handover between cells.
In 3GPP LTE, the preamble sequences in Step 1 are generated from one or several root Zadoff-Chu (ZC) sequences, as specified in 3GPP TS 36.211 v12.4.0. Basically there are a number of available sequences for a UE to select for conducting the random access in one cell. For example, in an LTE system, there are 64 sequences in each cell that can be used for random access. Each time when a UE is about to conduct the random access, one sequence out of the 64 sequences is selected. A collision may occur if several UEs are selecting the same sequence which could result in a random access failure for some or all UEs.
According to the contention-based procedure, a UE may initiate random access by transmitting a random access preamble to a base station, e.g. eNB in LTE, via a Physical Random Access Channel (PRACH). This preamble may also be referred to as a PRACH preamble hereafter.
Once the preamble is detected by the eNB in a time-frequency slot, the eNB would send a Random Access Response (RAR) on a Physical Downlink Shared Channel (PDSCH) in Step 2, and address it with a Random Access Radio Network Temporary Identifier (RA-RNTI), which conveys the identity of the detected preamble, a timing alignment instruction to synchronize subsequent uplink transmissions from the UE, an initial uplink resource grant for transmission of the Step 3 message, and an assignment of a temporary Cell Radio Network Temporary Identifier (C-RNTI). Then in Step 3, the UE would convey an actual random access procedure message, such as a Radio Resource Control (RRC) connection request, tracking area update, or scheduling request to the eNB. Finally, in Step 4, a contention resolution message will be sent by the eNB.
With further development of radio communications, much higher carrier frequencies and more antenna elements are adopted. In order to construct a random access preamble which is robust against phase noise and a frequency error or offset for the high carrier frequency and reduce hardware complexity with multiple antennas, a new random-access preamble format has been proposed in some discussions for the next generation communications system, e.g. 5G system including millimeter wave (mmW) networks, and concept development, e.g. in a PCT application PCT/EP2014/055898. The proposed preamble sequence is constructed by repeating a short sequence multiple times, the length of each short sequence equal to that of a symbol, e.g. a Single Carrier Frequency Division Multiple Access (SC-FDMA) symbol transmitted in uplink for carrying user data, and thus a preamble detector at the base station may reuse existing Fast Fourier Transforming (FFT) modules configured for other uplink channels, e.g. a Physical Uplink Shared Channel (PUSCH), and signals as shown in FIG. 1, which schematically illustrates an existing procedure for random access preamble detection in the prior art. In this way, the amount of dedicated random-access related processing and hardware support is significantly reduced for multi-antenna systems, and the detector is also robust against inter-carrier interference from other uplink channels and signals.
As illustrated in FIG. 1, the received signal comprises a preamble sequence of 14 repeating short sequences, each corresponding to a symbol for carrying user data. 12 FFT modules at the detector may be used for converting corresponding short sequences into the frequency domain before the following matched filters (MFs). Then coherent accumulation of all outputs of the MFs is applied before inverse FFT (IFFT) processing. Finally, based on absolute square values of the IFFT outputs, the preamble may be detected together along with round trip time estimation.
Since one of the main design targets for the next generation communications systems is to work on the high frequency spectrum, such as 15 GHz or even higher, hardware impairments, such as phase noise, would become much more significant than in the current systems working on the low frequency spectrum. For example, assuming the phase noise from the hardware in a UE introduces a frequency error of 0.1 ppm (part per million), there would be 1.5 kHz frequency error or offset on the 15 GHz carrier frequency. A Doppler shift may further increase this frequency error or offset when the UE is moving. Thus, a phase rotation of the received PRACH preamble over the preamble length, for example the total length of 14 short sequences in FIG. 1, increases with the increasing frequency error or offset, which results in a restriction on the coherent accumulation time of the detector at the base station. This phase rotation in combination with the coherent accumulation results in a high access failure rate, i.e. low receiving sensitivity. It is well known that the low PRACH receiving sensitivity will limit the system coverage, and thus degrade the system performance.